The commercial production of alcohol by distillation has been in widespread operation for many years. Control systems for assuring product quality within reasonable efficiency limits have paralleled the growth of this industry. In the past, most of the alcohol distilled was for beverage purposes, and accordingly, there was no crucial requirement for a dehydrated end product, thereby alleviating to some extent both the energy required to distill the alcohol and the need for tight controls over the process. However, the rising cost of energy has focused attention on the need for better product optimization of energy intensive (endothermic) processes such as distillation through the application of dynamic control strategies.
Attempts to alleviate energy dependence on petroleum based fuels have been directed to the use of renewable energy sources. One such technique involves the production of ethanol from grain for blending with gasoline to form the motor fuel "gasohol". To be effective as an alternative energy source, the process by which the ethanol is produced must minimize energy consumption so as to achieve "a net energy gain", and moreover final stage ethanol should be essentially anhydrous.
The net effect of these circumstances is to place an increased emphasis on the systems controlling the operation of the distillation unit so that the desired end products can be produced with minimum energy drain. In the specific example of pure ethanol production, the problems are compounded because, as is well known, ethanol and water form an azeotrope whose water percentage is unacceptable for use in making gasohol. To separate the components of the azeotrope, further process steps over and above conventional distillation are required involving the expenditure of more energy and rather expensive entrainer substances. All of this adds detrimentally to the overall cost of making the finished product to desired specifications.
Although a variety of techniques have been applied in the past for the control of alcohol distillation columns, these methods usually rely on simple temperature measurements in conjunction with single-loop flow and level controllers. Generally these loops respond to conditions occurring external of the columns which are used to predict internal column activity; however, such control loops are slow responding and, most often, not truly representative of the dynamics within the column. Such techniques have thus had difficulties in maintaining effective control over the process, most notably during upset process conditions. This results in particularly significant deficiencies in bringing about unacceptable alcohol losses, high energy consumption, contaminated end product or combinations of the above. Therefore, at present there is a pressing need for an improved process for controlling the production of anhydrous alcohol through a multi-stage process involving a combination of conventional distillation and azeotropic distillation.